Electrical transformation lines as components of matching networks are frequently realized in a multilayer ceramic substrate, in which, as mentioned supra, additional elements can be integrated. Transformation lines are employed, e.g., in front-end modules for mobile communications terminal devices, where they may used as components of PIN diode switches, with a requirement of a phase shift of, e.g., c. 90°. Another criterion is that such a transformation line must be optimally matched under conditions of prescribed source and load impedances. In another application, mentioned here as an example, a transformation line is part of a duplexer in a terminal device of a mobile communications system, wherewith said line connects the antenna to the sending path and the receiving path of the terminal device.
Another criterion applied to transformation lines, particularly in terminal devices of mobile communications systems, is minimization of area and space requirements. E.g., in a front-end module, the external dimensions are substantially less than the fraction of the wavelength in the ceramic substrate in which the phase shift is to be accomplished. Because the prescribed phase shift can occur only with a conductor which has a certain geometric length, currently employed transformation lines have a bent-over configuration (e.g. in a zigzag or Greek fret pattern) and often also a multilayer configuration, whereby individual conductor segments overlap, giving rise to capacitive and inductive cross-coupling between different segments of the conductor, as a result of the bent-over configuration as well as of the multilayer configuration. This leads to changes in the matching, and to an additional phase shift beyond that of an ideal line which has the same geometric length but which is configured in a single layer and without bends. Further, the area available, and the position of connecting points (or terminals) at which the transformation line is connected to the component or to the rest of the matching network, cannot be chosen arbitrarily, because these choices depend on the other components of the circuit parts which are being integrated.
An advantageous means of realizing a transformation line is with the use of a so-called “tri-plate” conductor wherein a conductor, e.g. a bent-over (e.g. in a Greek fret pattern) conductor, is laid out between two grounded shielding layers, which are namely metallized plates, with a ceramic layer separating each such shielding layer from the conductor. The distance between the upper and lower grounded shielding plates influences the characteristic impedance, and is chosen accordingly. One cannot arbitrarily choose the thicknesses of the ceramic layers, because technical constraints apply, including the need to integrate with other elements in a common substrate; rather, the said thicknesses must be selected from a limited number of available suitable layer thicknesses, and thus one cannot achieve an optimal matching.
In a known space-saving transmission line, the conductor is laid out in, e.g., a bent-over Greek fret pattern and is realized in two layers. A symmetrical [sic] arrangement of the two planes in which the conductor is laid out is chosen, so that the characteristic impedance of the conductor in the two conductor planes is the same, and corresponds to the impedance of the source and the load. The cross-coupling between the individual segments of the conductor is minimized in that mutually parallel segments of the conductor are sufficiently separated, with the distance between them being as a rule greater than the width of the conductor. The cross-coupling between conductor segments in different conductor planes is reduced in that superposed segments in the two planes are disposed at mutual right angles, or conductor segments in one plane are laid out between the projections of conductor segments from the other plane. In order to increase the phase shift of the transmission line, the geometric length of the conductor may be increased. In situations where the available area is limited, this is only possible if the individual segments of the conductor can be laid out more densely (moved closer together). However, this causes increased cross-coupling of parts of the conductor, detracting from the matching between source and load.